1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to apparatus and methods for synchronization of laser firing and motion systems during laser shock peening.
2. Description of Related Art
Laser shock peening (LSP) or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high and low power pulsed lasers to produce an intense shock wave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477 entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957 entitled “Material Properties”. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No. 5,736,965 entitled “On The Fly Laser Shock Peening”; U.S. Pat. No. 5,591,009 entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,531,570 entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447 entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329 entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328 entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee.
High energy laser beams, from about 20 to about 50 Joules, or low energy laser beams, from about 3 to about 10 Joules, have been used and other levels are contemplated. See, for example, U.S. Pat. No. 5,674,329 (Mannava et al.), issued Oct. 7, 1997 which discloses LSP processing using high energy lasers and U.S. Pat. No. 5,932,120 (Mannava et al.), issued Aug. 3, 1999 which discloses LSP processing using low energy lasers. Low energy laser beams can be produced using different laser materials such as neodymium doped yttrium aluminum garnet (Nd YAG), Nd:YLF, and others.
Many laser shots must be taken in order to cover a significant area with laser shock peening and, thus, it is desirable to have a continuous LSP process requiring synchronization of laser firing and motion steps and systems. The motion must be effected between the workpiece being laser shock peened and the laser beams so that the laser hits the workpiece at different locations to cover the entire area to be laser shock peened with laser shock peened spots. Without synchronization, the requirement for repeatability needed for laser shock peened spot placement from one pass or layer of spots to the next cannot be met, when processing with a continuous motion. Laser shock peened spots on one layer or pass must hit between spots on previous layers or passes.
Laser shock peening has typically included moving a workpiece to fixed position, the motion is stopped, a laser is fired, and then the motion of the part is started again to reposition the workpiece for the next laser firing. This stop and start motion is repeated until the laser shock peened area or patch is completed. On the fly laser shock peening is disclosed in U.S. Pat. No. 5,756,965 which includes using a fixed constant laser pulse rate with a fixed feed rate motion system that produces overlapping row of laser beam spots. It is desirable to more precisely locate the laser shock peened spot needed for processing non-overlapping shots on one layer and placing laser shock peened spots between existing spots on subsequent layers which overlap laser shock peened spots of an earlier layer.